to the Antarctic Begions. 1 79 



vilely to calculate its results for given localities, however numerous, 

 and thereby load its apparent errors with the real errors, both of ob- 

 servation and of local magnetism; but to compare the totality of the 

 lines in our charts with the corresponding lines, as they result from 

 the formulae to be tested, when their general agreement or disagree- 

 ment will not only show how far the latter truly represent the facts, 

 but will furnish distinct indications of the modifications they require. 



Unfortunately for the progress of our theories, however, we are 

 yet very far from possessing charts even of that one element, the 

 Declination, most useful to the navigator, which satisfy these requi- 

 sites ; while as respects the others (the Inclination and Intensity) 

 the most lamentable deficiencies occur, especially in the Antarctic 

 regions. To make good these deficiencies by the continual practice 

 of every mode of observation appropriate to the circumstances in 

 which the observer is placed throughout the voyage, will be one of 

 the great objects to which attention must be directed. And first — 



At sea. — 'We are not to expect from magnetic observations made 

 at sea the precision of which they are susceptible on land. Never- 

 theless, it has been ascertained that not only the Declination, 

 but the Inclination and Intensity can be observed, in moderate cir- 

 cumstances of weather and sea, with sufficient correctness, to af- 

 ford most useful and valuable information, if patience be bestowed, 

 and proper precautions adopted. The total intensity, it is ascer- 

 tained, can be measured with some considerable degree of certainty 

 by the adoption of a statical method of observation recently de- 

 vised by Mr. Fox, whose instrument will be a part of the aj)- 

 paratus provided. And when it is recollected that but for such 

 observations the whole of that portion of the globe which is 

 covered by the ocean must remain for ever a blank in our charts, it 

 will be needless further to insist on the necessity of making a daily 

 series of magnetic observations, in all the three particulars above- 

 mentioned, whenever weather and sea will permit, an essential feature 

 in the business of the voyage, in both ships. Magnetic observations 

 at sea will, of course, be afl^ected by the ship's magnetism, and this 

 must be eliminated to obtain results of any service. To this end. 



First. Every series of observations made on board should be ac- 

 companied with a notice of the direction by compass of the ship's 

 head at the time. 



Secondly. Previous to sailing, a very careful series of the apparent 

 deviations, as shown by two compasses permanently fixed, (the one 

 as usual, the other in a convenient position, considerably more for- 

 ward in the ship,) in every position of the ship's head, as compared 

 with the real position of the ship, should be made and recorded, with 

 a view to attempt procuring the constants of the ship's action ac- 

 cording to M. Poisson's theory* ; and this process should be repeated 

 on one or more convenient occasions during the voyage; and, generally, 

 while at anchor, every opportunity should be taken of swinging round 

 the ship's head to the four cardinal points, and executing in each 

 position a complete series of the usual observations. 



Thirdly. Wherever magnetic instruments are landed and obser- 



* See Appendix A. 

 N 2 



